Process for preparing unsaturated polyester

ABSTRACT

The present invention relates to a process for preparing unsaturated polyester resin comprising itaconate ester units as reactive unsaturations, wherein the process comprises reacting at least itaconic acid and/or itaconic anhydride and at least one diol in the presence of benzoquinone and/or alkyl substituted benzoquinone, whereby the total amount of benzoquinone and alkyl substituted benzoquinone is at least 200 ppm.

The present invention relates to a process for preparing unsaturatedpolyester resin, suitable to be used in the manufacturing of structuralparts, comprising itaconate ester units as reactive unsaturations in theresin.

Unsaturated polyester resin compositions currently applied for obtainingstructural parts often contain considerable quantities of styrene ascopolymerizable reactive diluent. As a result of the presence ofstyrene, styrene may escape during the preparation and curing, but alsoeven during the envisaged long-term use thereof, and causes anundesirable odour, and possibly even also toxic effects. Thus there is adesire to reduce the styrene emission. Due to the fact that theunsaturated polyester resin is only copolymerizable, the amount ofstyrene in styrene containing resins can not be reduced further withoutdetrimentally affecting the properties of the cured composites.Consequently there is a need for homopolymerizable unsaturated polyesterresins. As used herein, an unsaturated polyester resin comprisingitaconate ester units as reactive unsaturations is consideredhomopolymerizable in case at least 25 wt. % of the unsaturateddicarboxylic acid building blocks are capable of homopolymerization,i.e. are itaconic acid building blocks.

It has however been found that homopolymerizable unsaturated polyesterresins tend to gel (i.e. to polymerize) during its preparation, inparticular when applying the standard UP synthesis procedure in whichall the raw materials for the resin are mixed and condensed at highertemperatures.

For unsaturated polyesters to be used in the manufacturing of structuralparts, the unsaturated polyesters must be able to free radicalpolymerise (i.e to cure) after having diluted the resin with reactivediluent. This means that gelation during preparation of the unsaturatedpolyester resin need to be avoided.

WO-A-9727253 describes the synthesis of itaconic based powder coatingresins. In this publication, the standard UP synthesis procedure, inwhich all the raw materials for the resin are mixed and condensed athigher temperatures, is not applied. The homopolymerizable itaconicbased unsaturated polyesters have been prepared by either using theprepolymer approach, in which a hydroxyl functional polymer or oligomeris modified in the second step with itaconic acid and condensed to lowacid values, or by using the anhydride approach, in which hydroxylfunctional polymers are modified with itaconic anhydride resulting inhigh acid value resins.

FR-A-1295841 discloses the preparation of unsaturated polyestercomprising itaconate ester units as reactive unsaturations applying thestandard unsaturated polyester synthesis procedure in the presence ofhydroquinone. One of the disadvantages of using hydroquinone is that itmay influence the curing rate of the resin. It is well known thatinhibitors may have an influence on the cure characteristics of theresin. Therefore, it is desired to have an alternative inhibitor thatcan be applied for preparing the resin. Furthermore, the inhibitors usedin the resin synthesis may have a negative impact on the mechanicalproperties of the cured resin. In view of this, it is also desired tohave an alternative inhibitor that can be applied for preparing theresin. Therefore, there is a strong desire to have alternativeinhibitors that can be used for preparing unsaturated polyesters withitaconate ester units.

The object of the present invention is to provide an alternativeinhibitor that can be applied in the standard unsaturated polyestersynthesis procedure for preparing unsaturated polyester comprisingitaconate ester units as reactive unsaturations, whereby the viscositiesof the prepared resins are similar or even lower and the mechanicalproperties of the cured resins are similar or even better.

This object has been surprisingly been achieved in that the processcomprises reacting at least itaconic acid and/or itaconic anhydride andat least one diol in the presence of benzoquinone and/or alkylsubstituted benzoquinone, whereby the total amount of benzoquinone andalkyl substituted benzoquinone is at least 200 ppm. (relative to thetotal amount of raw materials used to prepare the unsaturatedpolyester).

This is the more surprising since applying 100 ppm of benzoquinone oralkyl substituted benzoquinone results in gelation of the resin, whilewhen applying 100 ppm hydroquinone no gelation occurs.

An additional advantage of the process of the present invention is thatthe mechanical properties of a cured object based on a resin preparedwith the process according to the invention can be improved, inparticular HDT.

The process of the present invention preferably comprises:

-   -   (i) charging the reactor with itaconic acid and/or itaconic        anhydride and optionally other dicarboxylic acids, at least one        diol and an inhibitor,    -   (ii) heating till a temperature of from 180 to 200° C. until the        acid value is below 60,    -   (iii) cooling the resin, preferably to from 20 to 120° C., and    -   (iv) optionally diluting the resin with reactive diluent,    -   (v) wherein the inhibitor is benzoquinone and/or alkyl        substituted benzoquinone and whereby the total amount of        benzoquinone and alkyl substituted benzoquinone is at least 200        ppm. As used herein, at least means higher than or equal to.

Preferably the total amount of benzoquinone and alkyl substitutedbenzoquinone inhibitor is at least 300 ppm, more preferably the amountof benzoquinone and alkyl substituted benzoquinone inhibitor is at least400 ppm. The process may be effected in the presence of an additionalinhibitor. The process of the present invention is preferably effectedin the presence of at most 2000 ppm, more preferably at most 750 ppm ofinhibitor.

In a preferred embodiment, the process is effected in the presence ofalkyl substituted benzoquinone, as this may result, compared to the useof benzoquinone, in a lower viscosity of the prepared resin.

In another preferred embodiment, the process is effected in the presenceof benzoquinone and alkyl substituted benzoquinone. In this preferredembodiment, the process is preferably effected in the presence of atleast 200 ppm alkyl substituted benzoquinone and in the presence of atleast 200 ppm benzoquinone. It has been found that using the combinationof benzoquinone and alkyl substituted benzoquinone has a synergeticeffect on the decrease of viscosity of the prepared resin, whilemaintaining the thermal stability of the cured resin.

Non limited examples of alkyl substituted benzoquinone are 2-methylbenzoquinone, 2,3-dimethyl benzoquinone, 2,5-dimethyl benzoquinone,2,6-dimethyl benzoquinone, trimethyl benzoquinone, tetramethylbenzoquinone, 2-tert.butyl benzoquinone, 2-tert.butyl 6-methylbenzoquinone. A preferred alkyl substituted benzoquinone is 2-methylbenzoquinone.

The unsaturated polyester according to the invention comprises itaconateester units as building blocks having the following structural formula.

The itaconic ester units (also referred to as itaconic acid buildingblocks) contain an ethylenic unsaturation that is able to copolymerizewith copolymerizable monomer in which the unsaturated polyester isdiluted. The unsaturated polyester according to the invention can bemanufactured by polycondensation of at least a diol and itaconic acid oritaconic acid anhydride as unsaturated dicarboxylic acid. Thepolycondensation may also be effected in the presence of otherdicarboxylic acids containing reactive unsaturations, such as forexample maleic acid or anhydride and fumaric acid and/or in the presenceof saturated aliphatic dicarboxylic acids or anhydrides, like forexample oxalic acid, succinic acid, adipic acid, sebacic acid and/or inthe presence of aromatic saturated dicarboxylic acids or anhydrides likefor example phthalic acid or anhydride and isophthalic acid. In thepolymerisation is further used a diol, such as for example 1,2-propyleneglycol, ethylene glycol, diethylene glycol, triethylene glycol,1,3-propanediol, dipropylene glycol, tripropylene glycol, neopentylglycol, hydrogenated bisphenol-A. or ethoxylated/propoxylated bisphenolA.

According to a preferred embodiment the molecular weight of the diol inthe unsaturated polyester resin is in the range from 60 to 250 Dalton.In a preferred embodiment at least part of the diols is selected fromeither isosorbide or 1,3-propanediol or both, preferably derived from anon fossil source like for example corn.

Preferably the total amount of diacids ranges from 20 to 80 mol % andthe total amount of diols ranges from 20 to 80 mol % (in which 100 mol %is sum diols and diacids).

In the unsaturated polyester resin according to the invention,preferably at least 25 wt. % of the dicarboxylic acid building blocksare itaconic acid building blocks. More preferably, at least 55 wt. % ofthe dicarboxylic acid building blocks in the unsaturated polyesteraccording to the invention are itaconic acid building blocks.

The unsaturated polyester according to the invention is preferablyhomopolymerizable. Preferably at least 25 wt. %, more preferably atleast 55 wt. %, of the unsaturated dicarboxylic acid building blocks areitaconic acid building blocks.

In a preferred embodiment, at least part of the itaconic acid oritaconic anhydride is derived from a non-fossil source, for example fromcorn.

The unsaturated polyester comprising itaconate ester units as reactiveunsaturations is preferably composed of from 33 to 66 mol % glycolmonomers, from 10 to 66 mol % itaconic acid monomers, from 0 to 65 mol %unsaturated diacid monomers (other than itaconic acid monomers, like forexample fumaric and maleic acid monomers) and from 0 to 65 mol % diacidsother than unsaturated diacid monomers, preferably from 0 to 50 mol %diacids other than unsaturated diacid monomers.

The acid value of the itaconate containing unsaturated polyester resinis preferably in the range from 30 to 100 mg KOH/g resin, morepreferably from 35 to 75 mg KOH/g resin. As used herein, the acid valueof the resin is determined titrimetrically according to ISO 2114-2000.

In one embodiment, the molar ratio of hydroxyl end groups and carboxylicacid end groups in the unsaturated polyester resin according to theinvention is in the range from 0.33 to 0.9. In another embodiment, themolar ratio of hydroxyl end groups and carboxylic acid end groups in theunsaturated polyester resin according to the invention is in the rangefrom 1.1 to 3.

The hydroxyl value of the itaconate containing unsaturated polyesterresin is preferably higher than 25 and more preferably higher than 40 mgKOH/g resin. As used herein, the hydroxyl value of the itaconatecontaining polyester is determined according to ISO 4629-1996.

Preferably, the molecular weight of the unsaturated polyester comprisingitaconate ester units as reactive unsaturations is at least 300 Dalton,preferably at least 500 Dalton and more preferably at least 750 Dalton.Preferably, the molecular weigth Mn of the unsaturated polyestercomprising itaconate ester units as reactive unsaturations is at most10.000 Dalton, more preferably at most 5000 Dalton. The molecular weight(Mn) is determined in tetrahydrofurane using GPC according to ISO13885-1 employing polystyrene standards and appropriate columns designedfor the determination of the molecular weights.

In a preferred embodiment of the invention, the molecular weight Mn isin the range from 750 to 5000 Dalton.

The glass transition temperature T_(g) of the unsaturated polyester ispreferably at least −70° C. and at most 100° C. In case the unsaturatedpolyester is applied for construction purposes, the glass transitiontemperature T_(g) of the unsaturated polyester resin present in theresin composition according to the invention is preferably at least −70°C., more preferably at least −50° C. and even more preferably at least−30° C. The T_(g) of the unsaturated polyester resin present in theresin composition according to the invention is preferably at most 70°C., more preferably at most 50° C. and even more preferably at most 30°C. As used herein, the T_(g) is determined by means of DSC (heating rate5° C./min).

The present invention also relates to a resin composition comprising anunsaturated polyester comprising itaconate ester units, benzoquinoneand/or alkyl substituted benzoquinone. In a preferred embodiment theresin composition comprises an unsaturated polyester comprisingitaconate ester units and alkyl substituted benzoquinone. In an evenmore preferred embodiment the resin composition comprises a unsaturatedpolyester comprising itaconate ester units, and benzoquinone and alkylsubstituted benzoquinone. Preferably, the alkyl substituted benzoquinoneis 2-methyl benzoquinone. Preferably, the resin composition comprisesbenzoquinone and alkyl substituted benzoquinone, whereby the totalamount of benzoquinone and alkyl substituted benzoquinone is at least200 ppm, more preferably at least 300 ppm (relative to the total resincomposition). Generally the amount of benzoquinone and alkyl substitutedbenzoquinone is at most 2000 ppm, more preferably at most 750 ppm. Theresin composition preferably comprises reactive diluent.

In one embodiment, the unsaturated polyester resin according to theinvention can be applied as a powder coating resin. The preparation ofpowder coating compositions is described by Misev in “Powder Coatings,Chemistry and Technology” (pp. 224-300; 1991, John Wiley) herebyincorporated by reference. Therefore the present invention also relatesto a powder coating composition comprising the unsaturated polyester asprepared with the process according to the invention. In case theunsaturated polyester according to the invention is applied in a powdercoating composition, the glass transition temperature T_(g) of theunsaturated polyester resin is preferably at least 20° C., morepreferably at least 25° C. and even more preferably at least 30° C. andat most 100° C., more preferably at most 80° C. and even more preferablyat most 60° C.

A common way to prepare a powder coating composition is to mix theseparately weight-out components in a premixer, heat the obtainedpremix, for example in a kneader, preferably in an extruder to obtain anextrudate, cool down the obtained extrudate until it solidifies andcrush it into granules or flakes that are further grinded to reduce theparticle size followed by appropriate classification to obtain a powdercoating composition of the right particle size. Therefore, the inventionalso relates to a process for the preparation of a powder coatingcomposition according to the invention comprising the steps of:

-   -   a. mixing the components of the powder coating composition to        obtain a premix    -   b. heating the obtained premix, preferably in an extruder, to        obtain an extrudate    -   c. cooling down the obtained extrudate to obtain a solidified        extrudate and    -   d. breaking the obtained solidified extrudate into smaller        particles to obtain the powder coating composition and        preferably comprising the further step of classifying the thus        prepared powder particles via a sieve and collect sieve fraction        with particle size below 90 μm.

The powder coating composition of the present invention may optionallycontain the usual additives, such as for example fillers/pigments,degassing agents, flow agents, or (light) stabilizers. Examples of flowagents include Byk 361 N. Examples of suitable fillers/pigments includemetal oxides, silicates, carbonates or sulphates. Examples of suitablestabilizers include UV stabilizers, such as for example phosphonites,thioethers or HALS (hindered amine light stabilizers). Examples ofdegassing agents include benzoin and cyclohexane dimethanol bisbenzoate.Other additives, such as additives for improving tribo-chargeability mayalso be added.

In another aspect, the invention relates to a process for coating asubstrate comprising the following steps:

-   -   1) applying a powder coating composition according to the        invention to a substrate such that the substrate is partially or        fully coated with a coating.    -   2) heating the obtained partially or fully coated substrate for        such time and to such temperature such that the coating is at        least partially cured.

The powder coating composition of the present invention may be appliedusing the techniques known to the person skilled in the art, for exampleusing electrostatic spray or electrostatic fluidized bed.

In a preferred embodiment of the invention, the process according to theinvention further comprises the step of diluting the unsaturatedpolyester resin comprising itaconate ester units with one or morereactive diluents to obtain a resin composition suitable to be appliedfor construction purposes. In a preferred embodiment, the unsaturatedpolyester comprising itaconate ester units is diluted in styrene,dimethyl itaconate and/or a methacrylate.

The amount of such reactive diluent in the resin composition accordingto the invention is usually in the range from 5 to 75 wt. %, preferablyin the range from 20 to 60 wt. %, most preferably in the range from 30to 50 wt. % (relative to the total amount of unsaturated polyester andreactive diluent present in the resin composition). The diluent will beapplied, for instance, for lowering of the viscosity of the resincomposition in order to make handling thereof more easy. For claritypurpose, a reactive diluent is a diluent that is able to copolymerizewith the unsaturated polyester resin. Ethylenically unsaturatedcompounds can be advantageously used as reactive diluent. Preferably,styrene, dimethyl itaconate and/or a methacrylate containing compound isused as reactive diluent. In one embodiment of the invention, styrene,α-methylstyrene, 4-methylstyrene, (meth)acrylate containing compounds,N-vinylpyrrolidone and/or N-vinylcaprolactam is used as reactivediluent. In this embodiment, styrene and/or (meth)acrylate containingcompound is preferably used as reactive diluent and more preferably(meth)acrylate containing compound is used as reactive diluent. Inanother embodiment, itaconic acid or an ester of itaconic acid is usedas reactive diluent. In a more preferred embodiment, the reactivediluent comprises an ester of itaconic acid and at least anotherethylenically unsaturated compound, such as for example styrene,α-methylstyrene, 4-methylstyrene, (meth)acrylates, N-vinylpyrrolidoneand/or N-vinylcaprolactam. In this embodiment, the resin compositionpreferably comprises an ester of itaconic acid as reactive diluent andstyrene as reactive diluent or a methacrylate containing compound asreactive diluent. A preferred ester of itaconic acid is dimethylitaconate.

The resin composition preferably further comprises a co-initiator forthe radical curing of the resin composition, in an amount of from0.00001 to 10 wt % (relative to the total amount of unsaturatedpolyester and reactive diluent). A preferred co-initiator is an amine ora transition metal compound.

The amine co-initiator that may be present in the composition ispreferably an aromatic amine and even more preferably a tertiaryaromatic amine. Suitable accelerators include N,N-dimethylaniline,N,N-diethylaniline; toluidines and xylidines such asN,N-diisopropanol-para-toluidine; N,N-dimethyl-p-toluidine;N,N-bis(2-hydroxyethyl)xylidine and -toluidine. The amount of amine inthe resin composition (relative to the total amount of unsaturatedpolyester and reactive diluent) is generally at least 0.00001 wt. % andpreferably at least 0.01 wt. % and more preferably at least 0.1 wt. %.Generally, the amount of amine in the resin composition is at most 10wt. %, preferably at most 5 wt. %.

Examples of suitable transition metal compounds as co-initiator arecompounds of a transition metal with an atomic number of in the rangefrom 22 to 29 or with an atomic number in the range from 38 to 49 orwith an atomic number in the range from 57 to 79, such as vanadium,iron, manganese, copper, nickel, molybdenum, tungsten, cobalt, chromiumcompounds. Preferred transition metals are V, Cu, Co, Mn and Fe.

After having diluted the unsaturated polyester according to theinvention with reactive diluent, additional radical inhibitors may beadded. These radical inhibitors are preferably chosen from the group ofphenolic compounds, benzoquinones, hydroquinones, catechols, stableradicals and/or phenothiazines. The amount of radical inhibitor that canbe added may vary within rather wide ranges, and may be chosen as afirst indication of the gel time as is desired to be achieved.

Suitable examples of radical inhibitors that can be used in the resincompositions according to the invention are, for instance,2-methoxyphenol, 4-methoxyphenol, 2,6-di-t-butyl-4-methylphenol,2,6-di-t-butylphenol, 2,4,6-trimethyl-phenol,2,4,6-tris-dimethylaminomethyl phenol,4,4′-thio-bis(3-methyl-6-t-butylphenol), 4,4′-isopropylidene diphenol,2,4-di-t-butylphenol, 6,6′-di-t-butyl-2,2′-methylene di-p-cresol,hydroquinone, 2-methylhydroquinone, 2-t-butylhydroquinone,2,5-di-t-butylhydroquinone, 2,6-di-t-butylhydroquinone,2,6-dimethylhydroquinone, 2,3,5-trimethylhydroquinone, catechol,4-t-butylcatechol, 4,6-di-t-butylcatechol, benzoquinone,2,3,5,6-tetrachloro-1,4-benzoquinone, methylbenzoquinone,2,6-dimethylbenzoquinone, napthoquinone,1-oxyl-2,2,6,6-tetramethylpiperidine,1-oxyl-2,2,6,6-tetramethylpiperidine-4-ol (a compound also referred toas TEMPOL), 1-oxyl-2,2,6,6-tetramethylpiperidine-4-one (a compound alsoreferred to as TEMPON), 1-oxyl-2,2,6,6-tetramethyl-4-carboxyl-piperidine(a compound also referred to as 4-carboxy-TEMPO),1-oxyl-2,2,5,5-tetramethylpyrrolidine,1-oxyl-2,2,5,5-tetramethyl-3-carboxylpyrrolidine (also called3-carboxy-PROXYL), galvinoxyl, aluminium-N-nitrosophenyl hydroxylamine,diethylhydroxylamine, phenothiazine and/or derivatives or combinationsof any of these compounds.

Advantageously, the amount of radical inhibitor in the resin compositionaccording to the invention is in the range from 0.0001 to 10% by weight,(relative to the total amount of unsaturated polyester and reactivediluent). More preferably, the amount of inhibitor in the resincomposition is in the range from 0.001 to 1% by weight. The skilled manquite easily can assess, in dependence of the type of inhibitorselected, which amount thereof leads to good results according to theinvention.

The present invention further relates to a process for radically curingthe resin composition according to the invention, wherein the curing iseffected by adding an initiator to the resin composition as describedabove. Preferably, the curing is effected at a temperature in the rangefrom −20 to +200° C., preferably in the range from −20 to +100° C., andmost preferably in the range from −10 to +60° C. (so-called coldcuring). The initiator is a photoinitiator, a thermal initiator and/orredox initiator.

As meant herein, a photo initiator is capable of initiating curing uponirradiation Photo initiation is understood to be curing usingirradiation with light of a suitable wavelength (photo irradiation).This is also referred to as light cure.

A photo-initiating system may consist of a photo initiator as such, ormay be a combination of a photo initiator and a sensitizer, or may be amixture of photo initiators, optionally in combination with one or moresensitizers.

The photo initiating system that can be used in the context of thepresent invention can be chosen from the large group of photo-initiatingsystems known to the skilled person. A vast number of suitable photoinitiating systems, can be found in, for instance, Volume 3 of“Chemistry and Technology of UV and EB Formulations”, 2^(nd) Edition, byK. Dietliker and J. V. Crivello (SITA Technology, London; 1998).

The thermal initiator can be selected from azo compounds like forexample azo isobutyronitril (AIBN), C—C labile compounds like forexample benzopinacole, peroxides, and mixtures thereof. The thermalinitiator is preferably an organic peroxide, or a combination of two ormore organic peroxides.

The redox initiator is preferably an organic peroxide in combinationwith at least one of the above mentioned co-initiators. Examples ofsuitable peroxides are, for instance, hydroperoxides, peroxy carbonates(of the formula —OC(O)OO—), peroxyesters (of the formula —C(O)OO—),diacylperoxides (of the formula —C(O)OOC(O)—), dialkylperoxides (of theformula —OO—), etc.

The present invention further also relates to objects and parts preparedfrom such unsaturated polyester resin or to objects and structural partsprepared from unsaturated polyester resin compositions as describedabove, by curing with an initiator as described above. As used herein,structural resin compositions are capable of providing structural parts.Generally such resin compositions are non-aqueous systems. They containat most 5% by weight of water, mainly resulting from the reactionsduring resin preparation. As meant herein, structural parts areconsidered to have a thickness of at least 0.5 mm and appropriatemechanical properties. End segments where the resin compositionsaccording to the present invention can be applied are for exampleautomotive parts, boats, chemical anchoring, roofing, construction,containers, relining, pipes, tanks, flooring, windmill blades.

The present invention in particular relates to cured objects orstructural parts obtained by curing of a resin composition according tothe invention with an initiator, preferably comprising a peroxide.According to one embodiment, the curing is preferably effected bymoulding, more preferably the curing is effected by compression mouldingto obtain in particular a SMC or BMC part. The moulding is preferablyeffected at a temperature of at least 130° C., more preferably at least140° C.; and at a temperature of at most 170° C., more preferably of atmost 160° C.

The invention is now demonstrated by means of a series of examples andcomparative examples. All examples are supportive of the scope ofclaims. The invention, however, is not restricted to the specificembodiments as shown in the examples.

Standard Resin Synthesis

The diols, diacids and/or anhydrides, optionally inhibitor and catalystwere charged in a vessel equipped with a packed column, a temperaturemeasurement device and inert gas inlet. The mixture was heated slowly byusual methods to 200° C. The mixture was kept at 200° C. until thedistillation of water stopped. The packed column was removed and themixture was kept under reduced pressure until the acid value reached avalue below 50 mg KOH/g resin. Then the vacuum was relieved with inertgas, and the mixture was cooled down to 130° C. or lower. The solid UPresins were obtained in this way. Next the solid resin was dissolved ina reactive diluent at temperatures below 80° C.

Monitoring of Curing

Curing was monitored by means of standard gel time equipment. This isintended to mean that both the gel time (T_(gel) or T_(25->35° C.)) andpeak time (T_(peak) or T_(25->peak)) were determined by exothermmeasurements according to the method of DIN 16945 when curing the resinwith the peroxide as indicated.

Mechanical Property Determination

For the determination of mechanical properties 4 mm castings wereprepared. After 16 hrs the castings were released from the mould andpostcured using 24 hr at 60 C followed by 24 hr at 80 C.

Mechanical properties of the cured objects were determined according toISO 527-2. The Heat Distortion Temperature (HDT) was measured accordingto ISO 75-A.

The viscosity of the dissolved resin was determined at 23° C. using aphysica instrument. The viscosity of the neat resin was determined at125° C. using a cone and plate setup (Brookfield CAP200+ cone 3).

COMPARATIVE EXPERIMENTS A-K AND EXAMPLE 1-2

Resins were prepared via the standard synthesis procedure with theingredients and inhibitors as listed in table 1.

TABLE 1 A 1 B 2 C D E F G H I J K Itaconic acid 732 732 732 732 732 732732 732 732 732 732 732 732 Propylene glycol 471 471 471 471 471 471 471471 471 471 471 471 471 none Benzoquinone 0.12 0.6 (100 (500 ppm) ppm)2-methyl benzoquinone 0.12 0.6 (100 (500 ppm) ppm)2,3,5,6-tetrachloro-1,4- 0.12 0.6 benzoquinone (100 (500 ppm) ppm)hydroquinone 0.12 0.6 (100 (500 ppm) ppm) Tempol 0.12 0.6 (100 (500 ppm)ppm) p-methoxy phenol 0.12 0.6 (100 (500 ppm) ppm) Acid value of 45 4545 45 140 103 45 44 45 46 45 44 45 the resin (mgKOH/g resin) Viscosity @125° C. (cone Gel 4.65 Gel 4.05 Gel Gel 9.22 4.25 Gel gel gel gel Geland plate) dPas

The comparative experiments A and B combined with the examples 1 and 2clearly demonstrate the fact that benzoquinone and alkylatedbenzoquinones in amounts higher than 100 ppm can be applied successfullyin the preparation of itaconate resins whereas employing amounts of 100ppm or lower results in a gel. This is surprising considering the factthat the other inhibitors either result in a gel both at low and highamounts (comp G-J) or result in a liquid resin at 125° C. both at lowand high amounts (comp E-F). Furthermore, comparing examples 1 and 2with comparative experiment F shows that the obtained resins havesimilar viscosities and that therefore benzoquinone and an alkylsubstituted benzoquinone are good alternatives to hydroquinone.

EXAMPLES 3-7 AND COMPARATIVE EXPERIMENT L

Resins were prepared via the standard synthesis procedure with theingredients and inhibitors as listed in table 2. The resins were curedusing 0.5 wt % of a cobalt solution

(NL-49P) followed by 2 wt. % Trigonox 44B as peroxide. The curing wasmonitored with the gel time equipment.

TABLE 2 Example Example Example Example Example 3 L 4 5 6 7 Itaconicacid 732 732 732 732 732 732 Propylene glycol 471 471 471 471 4711,3-propylene glycol 235.5 Ethylene glycol 192.1 Benzoquinone 0.6 0.30.3 2-methyl benzoquinone 0.6 0.3 0.3 0.3 Hydroquinone 0.6 2-methylhydroquinone 0.3 Viscosity @ 125° C. (cone 4.65 4.25 4.05 3.48 3.36 2.60and plate) dPa · s Acid value of the resin 45 44 45 45 45 45 (mgKOH/gresin) Reactive diluent styrene styrene styrene styrene styrene styreneSolid content % 65 65 65 65 65 65 Viscosity @23° C. mPa · s 976 816 770655 630 423 Tensile strength (MPa) 71 58 63 56 57 43 Tensile modulus(GPa) 3.6 2.6 3.5 3.4 3.6 3.1 Elongation at break (%) 2.4 4.3 2.2 1.91.8 1.4 Flexural strength (MPa) 105 87 116 120 127 102 Flexural modulus(GPa) 3.9 2.8 3.8 3.5 3.7 3.3 Barcol Hardness 42 35 43 45 47 42 HDT (°C.) 98 85 98 98 101 103

From this table it is evident that the mechanical properties of thecured resins are improved by employing the inhibitors according to theinvention. Comparing example 3 or 4 with comparative experiment Lclearly shows this effect on tensile strength, tensile modulus, flexuralstrength, flexural modulus, Barcol hardness and especially the thermalstability as indicated by the HDT.

This is surprising since Table 1 shows that benzoquinone and alkylsubstituted benzoquinone are less efficient inhibitors compared tohydroquinone as with 100 ppm benzoquinone or alkyl substitutedbenzoquinone the resin gels (comparative experiments A and B).

Example 5 demonstrates that mixtures of benzoquinone with alkylatedbenzoquinones can be employed and that this combination results in asynergetic effect on the viscosity of the resin whilst maintaining thethermal stability. Example 6 demonstrates that mixtures of an alkylatedbenzoquinone with a hydroquinone can be employed an that an increase inflexural strength can be obtained compared to example 4. Example 7 showsthat various diols can be used.

1. Process for preparing unsaturated polyester resin comprisingitaconate ester units as reactive unsaturations, wherein the processcomprises reacting at least itaconic acid and/or itaconic anhydride andat least one diol in the presence of benzoquinone and/or alkylsubstituted benzoquinone, whereby the total amount of benzoquinone andalkyl substituted benzoquinone is at least 200 ppm.
 2. Process accordingto claim 1, wherein the total amount of benzoquinone and alkylsubstituted benzoquinone is at least 300 ppm.
 3. Process according toclaim 1, wherein the total amount of benzoquinone and alkyl substitutedbenzoquinone is at least 400 ppm.
 4. Process according to claim 1,wherein the process is effected in the presence of alkyl substitutedbenzoquinone.
 5. Process according to claim 1, wherein the process iseffected in the presence of alkyl substituted benzoquinone andbenzoquinone.
 6. Process according to claim 5, wherein the process iseffected in the presence of at least 200 ppm alkyl substitutedbenzoquinone and at least 200 ppm benzoquinone.
 7. Process according toclaim 1, wherein the alkyl substituted benzoquinone is 2-methylbenzoquinone.
 8. Process according to claim 1, wherein the processfurther comprises the step of diluting the resin with reactive diluent.9. Process according to claim 8, wherein the resin is diluted instyrene, dimethyl itaconate and/or a methacrylate.
 10. Resin compositioncomprising unsaturated polyester resin comprising itaconate ester unitsand benzoquinone and/or alkyl substituted benzoquinone.
 11. Resincomposition according to claim 10, wherein the resin compositioncomprises benzoquinone and/or alkyl substituted benzoquinone, wherebythe total amount of benzoquinone and alkyl substituted benzoquinone isat least 200 ppm.
 12. Resin composition according to claim 10, whereinthe alkyl substituted benzoquinone is 2-methyl benzoquinone.
 13. Resincomposition according to claim 10, wherein the composition furthercomprises reactive diluent.
 14. Cured object or structural part obtainedfrom a resin composition according to claim 10 by curing with aninitiator.
 15. Cured object or structural part according to claim 14,wherein the initiator comprises a peroxide.
 16. Use of the cured objector structural part of claim 14 in automotive parts, boats, chemicalanchoring, roofing, construction, containers, relining, pipes, tanks,flooring or windmill blades.
 17. Powder coating composition comprisingan unsaturated polyester resin composition according to claim 10.